Inspection of metal pipe or tubular members by magnetic means conventionally involves magnetizing the member to create a magnetic field which extends circumferentially and is characterized by lines of magnetic flux which extend either axially of the tubular member or generally perpendicular to its axis, dependent on the manner by which magnetism is induced. In many of the present systems, current flow through a wire coil positioned about the tubular member forms magnetic lines of flux through the opening of the coil which extend axially of the member under inspection. In other systems, current flows axially of the tubular member within the wall thereof so as to create a magnetic field, the lines of flux of which extend circumferentially about the tubular member in an orientation substantially perpendicular to the tubular member. The presence of structural flaws or anomalies in the wall of the tubular member, such as surface nicks or pits, cracks, voids, or various crystalline discontinuities, disturbs the uniformity of a magnetic field in the wall of the tubular member. Accordingly, the structural integrity of the tubular member and its relative freedom of such flaws may be inspected by sensing and detecting the magnetic field variations with sensors disposed closely adjacent the surface of the tubular member.
The sensing of variations in a magnetic field in a pipe wall is customarily achieved by passing or moving an induction coil or similar device through the magnetic field and any magnetic field variations to induce voltages in the coil indicative of the magnetic field variations. The voltages or voltage signals may then be transmitted to appropriate recording and processing equipment. For optimum detection sensitivity and for detection of very small cracks in a pipe wall which can be a few thousandths of an inch in depth, it is necessary that the detecting or sensor coil be placed closely adjacent the exterior surface of the tubular member. Normally the adjacent coil is disposed from 0.002 inches (0.051 mm) to 0.020 inches (0.51 mm) relative the surface of the tubular member with the axis of the coil substantially perpendicular to the surface. The sensor coils are mounted in a support, conventionally referred to as a sensor shoe, which is provided with contact surfaces adapted to contact with the surface of the tubular member in a manner so that the sensing coil is supported closely adjacent the surface of the tubular member at an optimum distance or spacing. Normally, the coil is protected from direct contact with the surface of the pipe by means of a thin shim of magnetically transparent material.
Inspecting the surface of the tubular members requires that the inspection sensor be moved along the surface in a predetermined inspection path. In one widely used pipe inspection apparatus, a plurality of sensor shoes are applied to the surface of the pipe in circumferential spacing thereabout and each of the sensor shoes is moved relatively to the pipe in a circumferential helical path whereby the plurality of sensors provide more than 100 percent coverage of the pipe surface. The relative movement may be effected by moving the sensors longitudinally while rotating the sensor shoes around a stationary pipe or the pipe can be moved longitudinally while the sensors are rotated about the pipe. In any event, there is relative sliding movement between the sensor shoes and the pipe surface, which causes wear of the contact surfaces of a sensor shoe by extensive use. The shim also can come into contact with the pipe and can be similarly eroded. The wearing down of the contact surfaces and shim results in the coils being supported from the pipe surface inside the optimum spacing range or possibly being damaged by eventual contact with the pipe. When either event occurs, a new sensor shoe is required.
Furthermore, sensor shoes are customarily designed for use with a specific diameter of pipe, and the contact surfaces of a sensor shoe are fixed in a permanent orientation such that when placed in contact with the surface of the pipe, the sensing coil is positioned for optimum detection sensitivity. Accordingly, a given sensor shoe is designed for inspection of only one diameter of pipe and is inappropriate for use as an inspection sensor for pipes of a different diameter since for such pipe diameters the sensing coil is supported at other than an optimum distance from the surface to be inspected.